Inorganic anti-corrosive pigments and method for the production thereof

ABSTRACT

Inorganic anti-corrosive pigments consist of surface-coated solids having a grain size of 0.1 to 75 μm. In order to improve the resistance of the paints and coatings containing such pigments, firstly Mn 3 (PO 4 ) 2  and secondly aluminium oxides and/or aluminium hydroxides are precipitated onto the solids.

The invention relates to inorganic anti-corrosive pigments, consistingof surface-coated finely divided solids having a grain size of 0.1 to 75μm, preferably 0.2 to 10 μm, a method for the preparation thereof andthe use thereof.

Inorganic fillers are solids that are present in a finely divided formand which differ from inorganic white pigments on account of theirrefractive index. Fillers have, as a rule, refractive indices <1.7 (withrespect to a vacuum), whilst white pigments have larger refractiveindices. A further distinctive feature can be seen in the fact thatthese materials are used for filling purposes, that is, they represent amain constituent in the respective mixture. Pigments, on the other hand,are used mainly in small quantities. However, the two limiting criteriaare not exact; on the one hand, a filler that has a refractive index of<1.7 can, in a suitably selected matrix, also have a certain coveringcapacity, that is, a property that is actually pigment-specific; on theother hand, certain fillers are also only used in small quantities. Thedividing line is therefore blurred.

Fillers have two tasks; on the one hand they are to bring down the costof a product in the conventional sense and ensure that, in comparisonwith products that are not filled, it has improved or additional, newproperties, and on the other hand the use of fillers is often tofacilitate the production of products.

In addition to the cost-lowering effect, fillers also have an influenceupon the rheology, that is, thickening and, if applicable, thixotropy,bytmeans of controlled adjustment of a time-dependent structuralviscosity, for example by means of talc, wollastonite and kaolin. Ingeneral, the flow behavior is to be affected in the case of lacquers,coatings, printing inks, liquid plastics materials and sealingcompounds. The visual properties are also affected by fillers; forexample the whiteness and covering capacity are improved when the truewhite pigment is replaced by inter alia synthetic silicates in dispersedyes. As a result of the use of mica, iron mica and barium sulphatefillers, it is possible to improve the corrosion protection of paintsand coatings against the influences of weather and chemicals.

In CZ-B-235 851 inter alia fillers are provided that are covered with alayer of Mn₃ (PO₄)₂ and with which lacquers are admixed for the purposesof protection against corrosion. JP-B-04 183 756 discloses a pigmentthat consists of sericite flakes the particles of which are covered witha layer of zinc oxide. The pigment particles are introduced intocoatings for steel sheets to provide protection against corrosion.DE-C-2 200 654 deals with fillers, such as talc, silicon dioxide,titanium dioxide or mixtures thereof with a metal molybdate coveringthat is precipitated on the particle surfaces thereof. These particlesare to be used as a corrosion-inhibiting additive in painting materials.According to U.S. Pat. No. 4,017,315, such a coating can additionallycontain metal phosphate as well. EP-A-0 505 086 describes a method inwhich zinc phosphate is precipitated onto lamellar and flaky particlesof synthetic iron oxide mica and these are introduced into coatings ofsteel components for protection against corrosion. In DE-C-2 245 995 afiller is proposed that is made from an inert core, consisting ofwollastonite, talc, mica, nepheline-syenite and/or feldspar, theparticle surfaces of which are covered with calcium phosphate, calciumphosphite and/or calcium borate.

This filler is to give paints and coatings an improved endurancestrength and hardness, and also a corrosion-inhibiting effect andtannin-absorbing properties.

The specialist world is constantly endeavouring to make paints,coatings, printing inks or the like resistant to the influences ofatmospheric substances and chemicals.

It is therefore the object of the present invention to make availableanti-corrosive pigments of the kind described at the beginning which, inaddition to the influences that they have on the rheological behaviorand also on the,visual and mechanical properties of paints and coatings,have a clearly improved corrosion-inhibiting effect upon the resistanceof paints and coatings.

This object is achieved by means of inorganic anti-corrosive pigmentscomprising surface-coated solids having a grain size of 0.1 to 75 μm,onto which firmly Mn₃(PO₄)₂ and secondly aluminum oxides and/or aluminumhydroxides are precipitated. More particularly, the surface-coatedsolids can have a grain size of 0.2 to 10 μm. In another aspect of theinvention, Mn₃(PO4)₂ is replaced totally or partly by Zn₃(PO₄)₂. Inaddition, phosphates and/or oxides of cerium and/or hydroxides of tincan be precipitated individually or severally onto the solids. In thatcase, the proportions of the substance, weight-related to the solid,amount to 0.1 to 15% manganese, 0.1 to 15% zinc, 0.1 to 15% Al₂O₃, 0.1to 5% CeO₂, and 0.1 to 10% SnO₂. More particularly, the proportions ofthe substances, weight-related to the solid, can amount to 0.1 to 3%manganese, 0.1 to 6% zinc, 0.1 to 6% Al₂O₃, 0.1 to 2.5% CeO₂, and 0.1 to5% SnO₂.

The solids can be natural silicate fillers, in particular feldsparzeolites, talc, pyrophyllite, kaolin, mica, muscovite, bentonite,wollastonite and tremolite; barium sulphate fillers, in particular heavyspar and blanc fixe; zinc pigments, in particular zinc, zinc oxide, zincsulphide and lithopones; iron mica; or aluminum oxide.

For preparing anti-corrosive pigments, the precipitation onto thesurface of the solids is effected from an aqueous solution, in which arecontained phosphorus compounds and soluble, inorganic and/or organicmetal compounds firmly of aluminum and secondly of manganese and/or ofzinc and, if applicable, thirdly of cerium and/or of tin, with theprecipitation being effected in a plurality of stages in which theindividual constituents are added.

The precipitation of the aluminum oxides and/or aluminum hydroxides canbe effected from an aqueous solution in which alkali aluminates,aluminum sulphates, aluminum hydroxides and aluminum halides are usedindividually or severally as aluminum compounds. Alkali stannates andcomparable organic salts can be used individually or severally asmanganese-, zinc-, tin- and cerium-nitrates, carbonates, hydroxides,sulphates, halides, acetates, and oxalates. One or more of the compoundsphosphoric acid, phosphorus pentoxide, alkali phosphate, alkali hydrogenphosphate, dihydrogen phosphate, pyrophosphate, triphosphate andpolyphosphate can be used as a phosphorus compound.

The solids, which can consist of natural silicate- and barium-sulphatefillers, can be ground before surface coating is carried out. Theprecipitation of the surface coating is carried out continuously ordiscontinuously. The precipitation can be effected in a plurality ofstages in succession or by means of simultaneous addition of a pluralityof constituents. The precipitation of the aluminum compounds can beeffected at the end.

The anti-corrosive pigments that are thus prepared can, after thesurface coating has been carried out, be subjected to thermalafter-treatment at temperatures of 400 to 1000° C., e.g., 600 to 900° C.The pigments can be ground up at the end of the preparation by means ofpinned disk mills or jet mills.

The anti-corrosive pigments can be used with a base of aqueous ororganic binding-agent systems that have solvents, in particular forpriming, base-lacquering, finish-lacquering and the like, in, forexample, automobile construction, railway construction, watercraftconstruction and aircraft construction and also for building protection.The anti-corrosive pigments can also be used for coatings, in particularfor electrophoretic lacquers for cathodic or anodic dip lacquering.

The inorganic anti-corrosive pigments formed in accordance with theinvention achieve, in particular, a clear increase in the resistance oflacquers, coatings and plastics materials to the influences of weatherand chemicals.

The preparation of the anti-corrosive pigments in accordance with theinvention is explained in greater detail in the following with referenceto the example of a barium sulphate carrier material. The BaSO₄ that isused can not only be natural heavy spar, but also freshly precipitatedsynthetic Baso₄ that has a different particle size and morphology.

An aqueous BaSO₄-suspension having a solids content of 21% BaSO₄ isheated to a temperature of 80° C. while subject to constant stirring.The pH values of the individual precipitation stages in the range of 4to 8 are adjusted during the after-treatment, as required, by means ofNaOH or H₂SO₄ and kept constant. The individual constituents—phosphoricacid and manganese sulphate solution and also subsequently sodiumaluminate solution—are precipitated stoichiometrically onto the surfaceof the carrier material, in this case onto BaSO₄, in a multi-stageprecipitation while subject to stirring and whilst maintaining pH andtemperature. After a ripening time of approximately 2 hours whilesubject to constant stirring at a predetermined temperature, theresultant product is filtered off, washed with deionized water, driedand, in the case of a material that is as finely divided as possible,subsequently ground in a jet mill.

Grinding in a jet mill is inapplicable if small plate-like carriermaterials are used. The product that is obtained can be subjected to anadditional optional annealing process.

The anti-corrosive pigments in accordance with the invention can beprepared by means of these method measures, that is, with the selectionof desired carrier materials and the fine-division or morphology thereofin combination with different, actively anti-corrosive surface coatings.

These anti-corrosive pigments are introduced into lacquer recipes indifferent pigment volume concentrations (PVC), that is, the ratio of thetotal volume of the anti-corrosive pigment in a product to the totalvolume of all the non-volatile constituents. An example of asolvent-based alkyd-coating system (data calculated in weight per centas a solid) is reproduced in the following:

Alkyd resin (short-oily) 20%, phenolic resin 5%, TiO₂ 8%, anti-corrosivepigment 10%, heavy spar 6%, talc 8%, anti-skin forming agent (oxine)0.4%, bentone 1%, desiccant 0.06%, remainder to give 100%: solventxylene, glycols, butyl acetate. The proportions of the pigments andfillers can be varied according to the requirements.

The constituents in this case are mixed or pre-dispersed by means of asolubilises and subsequently dispersed by way of a continuous mill. Interms of application techniques, salt-spray mist tests according to DIN51167, for example, can be carried out on this preparation bycentrifuging the coating system onto degreased sheets, 100×150×1 mm,drying the latter and subsequently introducing them into specialsalt-spray mist chambers.

What is claimed is:
 1. Inorganic anti-corrosive pigments, comprisingsolid particles having a grain size of 0.1 to 75 μm, a first precipitateselected from the group consisting of Mn₃(PO₄)₂, Zn₃(PO₄)₂ and mixturesthereof precipitated on the solid particles, and a second precipitateselected from the group consisting of aluminum oxides, aluminumhydroxides and mixtures thereof precipitated onto the solid particles.2. Inorganic anti-corrosive pigments according to claim 1, wherein thesolid particles have a grain size of 0.2 to 10 μm.
 3. Anti-corrosivepigments according to claim 1 wherein said first precipitate comprisesMn₃(PO₄)₂.
 4. Anti-corrosive pigments according to claim 1, furthercomprising at least one precipitate selected from the group consistingof phosphates and/or oxides of cerium and hydroxides of tin precipitatedonto the solid particles.
 5. Anti-corrosive pigments according to claim4, wherein the precipitates are provided, weight-related to the solid,in amounts of 0.1 to 15% manganese 0.1 to 15% zinc 0.1 to 15% Al₂O₃ 0.1to 5% CeO₂ 0.1 to 10% SnO₂.
 6. Anti-corrosive pigments according toclaim 5, wherein the precipitates are provided, weight-related to thesolid, in amounts of 0.1 to 3% manganese 0.1 to 6% zinc 0.1 to 6% Al₂O₃0.1 to 2.5% CeO₂ 0.1 to 5% SnO₂.
 7. Anti-corrosive pigments according toclaim 1, wherein the solid particles are natural silicate fillers. 8.Anti-corrosive pigments according to claim 1, wherein the solidparticles are barium sulphate fillers.
 9. Anti-corrosive pigmentsaccording to claim 1, wherein solid particles are zinc pigments. 10.Anti-corrosive pigments according to claim 1, wherein the solidparticles are iron mica.
 11. Anti-corrosive pigments according to claim1, wherein the solid particles are aluminium oxide.
 12. Anti-corrosivepigments according to claim 7, wherein the natural silicate fillers areselected from the group consisting of feldspar zeolites, talc,pyrophyllite, kaolin, mica, muscovite, bentonite, wollastonite andtremolite.
 13. Anti-corrosive pigments according to claim 8, wherein thebarium sulphate fillers are selected from the group consisting of heavyspar and blanc fixe.
 14. Anti-corrosive pigments according to claim 9,wherein the zinc pigments are selected from the group consisting ofzinc, zinc oxide, zinc sulphide and lithopones.
 15. Method for preparinganti-corrosive pigments comprising: providing solid particles having agrain size of 0.1 to 75 μm; and precipitating a first precipitateselected from the group consisting of Mn₃(PO₄)₂, Zn₃(PO₄)₂ and mixturesthereof and a second precipitate selected from the group consisting ofaluminum oxides, aluminum hydroxides and mixtures thereof onto thesurface of the solid particles, from an aqueous solution in whichphosphorus compounds and soluble, inorganic and/or organic metalcompounds of aluminum and soluble, inorganic and/or organic metalcompounds of manganese and/or of zinc are contained, with theprecipitation being effected in a plurality of stages.
 16. Methodaccording to claim 15, wherein the precipation of the aluminium oxidesand/or aluminium hydroxides is effected from an aqueous solution inwhich at least one aluminum compound selected from the group consistingof alkali aluminates, aluminium sulphates, aluminium hydroxides andaluminium halides is contained.
 17. Method according claim 15, where inthe soluble, inorganic and/or organic metal compounds of manganeseand/or zinc are selected from the group consisting of nitrates,carbonates, hydroxides, sulphates, halides, acetates, and oxalates ofmanganese and/or zinc.
 18. Method according to claim 15, wherein thephosphorous compounds are selected from the group consisting of thecompounds phosphoric acid, phosphorus pentoxide, alkali phosphate,alkali hydrogen phosphate, dihydrogen phosphate, pyrophosphate,triphosphate and polyphosphate.
 19. Method according to claim 15,wherein the solid particles comprise natural silicate- andbarium-sulphate fillers that are ground before precipitation is carriedout.
 20. Method according to claim 15, wherein the precipitation iscarried out continuously or discontinuously.
 21. Method according toclaim 15, wherein the precipitation of the aluminum compounds iseffected last.
 22. Method according claim 15 further comprising, afterthe precipitation has been carried out, subjecting the pigments tothermal after-treatment at temperatures of 400 to 1000° C.
 23. Methodaccording to claim 22, wherein the temperatures of the thermalafter-treatment are at 600 to 900° C.
 24. Method according to claim 15,further comprising grinding the pigments after precipitation by means ofpinned disk mills or jet mills.
 25. Method according to claim 15,further comprising precipitating at least one precipitate selected fromthe group consisting of phosphates and/or oxides of cerium andhydroxides of tin onto the solid particles.